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Old 27-04-2008, 07:57 AM
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ISO speeds

Hi all, a quick question.

How are ISO speeds for imaging determined?

I have noticed that some images are taken at ISO 800 while others are at 400, 200 etc.

I can see that there are subtle differences at different speeds but was wondering there are any general rules to follow to start off and then I can experiment from there.

Cheers
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Old 27-04-2008, 08:39 AM
shane shaw
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g'day mate. I am fairly new to photography especially astro Photography, however I believe that if you think of taking pictures of any subject at night with minimal light that 100 iso is just not going to work very well.

The smaller the F-stop number (or f/value), the larger the lens opening (aperture). this will allow more light in and give the the possibilty of using a lower iso.

So a high apeture (smaller opening) will allow less light in and require a higher iso. Just have to try and get a happy mix of the 2 and you will be fine.

As an example I went to a circus the other night and no flash was allowed and I took some really good photos just changing the iso, apeture and also shutter speed.

good luck I hope I have got it right
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Old 27-04-2008, 08:58 AM
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Hi Shane and thanks for the explanation.

As a scenario,
if I was imaging for 30 seconds @ f/10 and at a setting of say ISO 600. If I then changed to f/3.3 via a focal reducer I could then take the same 30 second image with a higher ISO speed say ISO 800 for example.

Does that sound right or have I gone off on a tangent.

Cheers
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Old 27-04-2008, 09:12 AM
Dennis
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Hi Ric

Speed is mainly related to the physical size of the “pixel” or “grain” which captures the incoming photon. Small pixels and small grains require a lot of light, but produce nice images with little noise when exposed optimally.

Large pixels and large grains capture photos more efficiently due to their larger surface area but tend to produce noisier or grainy images.

In the good old days of 35mm film SLR photography,
  • ISO 100 film was for brightly lit daylight scenes, such as sunny days at the beach, park, etc. This was a Slow Speed film.
  • ISO 400 film was used when taking photos at the daytime cricket match to allow fast shutter speeds to “freeze” the action. This was a Medium Speed film.
  • ISO 800 to 1600 was used at night events such as circuses, concerts, floodlit sporting events, etc. when flash was not practical. These were Fast Films.
In the 1990’s, advances in film technology meant that ISO 400 films were as good as older ISO 100 films. The “pixels” of film were silver halide grains which reacted (non-linearly) to light.
  • ISO 100 had a very fine grain which could produce prints up to say, 20”x24”.
  • ISO 400 had a more coarse grain, visible on 10”x8” prints
  • ISO 800 grain was quite visible even on 6”x4” prints.
Generally, lower ISO settings such as 100 to 400, produce less noisy images, with smoother tones, but require slower shutter speeds (risk of camera shake or subject movement) or wider apertures in order to grab sufficient incoming photons. These are your typical day at the beach scenes.

Cheers

Dennis
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Old 27-04-2008, 09:20 AM
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Thanks Dennis, all starting to come together now.

Cheers
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Old 27-04-2008, 09:21 AM
shane shaw
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Quote:
Originally Posted by Ric View Post
Hi Shane and thanks for the explanation.

As a scenario,
if I was imaging for 30 seconds @ f/10 and at a setting of say ISO 600. If I then changed to f/3.3 via a focal reducer I could then take the same 30 second image with a higher ISO speed say ISO 800 for example.

Does that sound right or have I gone off on a tangent.

Cheers
If you use a focal reducer then I would say you could then go down to say 400 or 200 with the iso. Because you have reduced the focal length your iso setting can go down. I believe Dennis has summed it up very well.

The higher focal length allows less light into the camera so if you used say 100 iso with a f/10 you would need a long exposure. with the f/3.3 if you used 100 iso you would need less exposure then the f/10.

Its hard to get a handle around it until you practice. I read up on it got text books and looked at various sites on the net and then practiced and i got it.
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Old 27-04-2008, 09:33 AM
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Thanks Shane, in the end it all boils down to experimentation.

Cheers
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Old 27-04-2008, 10:45 AM
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Here's some food for thought. If you expose for 10min at 400 iso, then for 5 min at 800 iso, your two images will have the same brightness. However, you will have gathered twice as many photons in the 10 min exposure, so you will get a much smoother image. There is no such thing as a free lunch.
Geoff
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